Abstract: The side impact test of a car was introduced, and a series of detection and analysis on the cracking problem of the bumper bar during the test was carried out, such as appearance observation, metallographic structure analysis, fracture analysis and three-point bending test. Finally, the basic failure cause of the part was found out and improvement measures were put forward. The results show that the material of the bumper bar is high strength weld pipe, and the microstructure sensitivity of the material is high after quenching and low temperature tempering. In the process of carbon dioxide protection welding between the bumper bar and the support, the local microstructure of the weld pipe was transformed under the effect of welding heat, weakening the mechanical properties. Crack initiated from the weak region during side impact test. By changing the welding seam layout between the bumper bar and the support from circumferential welding to longitudinal welding, the impact of welding heat was reduced. The subsequent parts have smoothly passed the impact test.